There is a continuing demand for accurate, low-cost sensors useful in a wide variety of applications, including equipment used in the home, as well as for security, military and transportation applications. For example, in the transportation field, the most prevalent sensor used for controlling traffic signals is a simple inductive loop which is buried beneath the pavement. While inductive loop sensors have proven to be effective in traffic control applications, they must be replaced not only when the loop malfunctions but also when the roadway is being resurfaced or an excavation takes place. Under these circumstances, the cost of loop replacement is of a lesser concern than the resulting disruption in traffic.
There have been suggestions for traffic signal controllers utilizing overhead sensors. See for example, the following U.S. patents: No. 3,167,739 to Girard et al; No. 3,436,540 to Lamorlett; No. 3,516,056 to Matthews; No. 3,532,886 to Kruger et al; No. 3,680,047 to Perlman; and No. 4,317,117 to Chasek.
Near-field sensors have also been utilized as intruder alarms and as automatic door operators. Examples of such arrangements are disclosed in the following U.S. patents: No. 3,605,082 to Matthews; No. 3,644,917 to Perlman; No. 3,719,938 to Perlman; No. 3,852,592 to Scoville et al; No. 3,972,021 to Leitz et al; and No. 4,433,328 to Saphir et al.